Change minutes to uint64 to avoid overflow in __wt_gen_drain

(CID 122000) There's potential for an integer overflow bug when calculating the time diff when waiting for generation drain. The comparison time_diff_ms > minutes * WT_MINUTE * WT_THOUSAND has a uint64 on the left, and a uint32 on the right. So, the RHS will overflow if minutes is ever greater than 71582. That's 49 straight days of generation drain - if that ever happens, there are likely other problems, but we should still fix it. There's a WT_ASSERT that minutes < 4, but we won't see that in a release build.

We can fix this by casting anything on the RHS to a uint64.